1. Field of the Invention
The present invention relates to a method of forming a titanium (Ti) film to be used as a contact metal film or an adhesion layer for a semiconductor device by a chemical vapor deposition (CVD) process.
2. Description of the Related Art
Recently, most semiconductor integrated circuits are fabricated in a circuit configuration of a multilevel structure to cope with market demand for high-density and large-scale integration. Accordingly, techniques for filling contact holes for the electrical connection of semiconductor devices formed in a lower layer to wiring lines formed in upper wiring layers, and via holes for the electrical connection of wiring lines in different wiring layers have become important.
Generally, aluminum (Al), tungsten (W), aluminum-base alloys and tungsten-base alloys are used for filling contact holes and via holes. If a contact hole (or a via hole) is filled with such a metal or an alloy, which is in direct contact with an aluminum (Al) wiring line or a silicon (Si) substrate in a lower layer, it is possible that an alloy of silicon and aluminum is produced in the boundary owing to the diffusion of Al and Si. Such an alloy has a high resistivity and is undesirable in view of need to reduce the power consumption and to increase the operating speed of integrated circuits If W or a W-base alloy is used for filling a contact hole, WF6 gas as a source gas for depositing W or the W-base alloy tends to deteriorate the electrical properties of the Si substrate.
To avoid such problems, a barrier layer is formed on surfaces defining contact holes or via holes before filling the contact holes and the via holes with the filling metal. Generally, a two-layer barrier layer consisting of a Ti film and a titanium nitride ( TiN) film is used as the barrier layer. It has been the usual way to form such a barrier layer by a physical vapor deposition (PVD) process. However, a PVD film has a poor coverage and is incapable of meeting the requirements of tight design rule, the reduction of the width of lines and diameter of openings and the increase of aspect ratio to meet the recent demand for the enhancement of the level of integration and the miniaturization of IC chips.
Recently, Ti films and TiN films have been formed by a CVD process capable of forming such films in a better film quality than the PVD process. The Ti film serving as a contact metal film is formed by a plasma CVD process. Usually, the plasma CVD process for forming a Ti film uses TiCl4 gas as a source gas, H2 gas and Ar gas. During the plasma CVD process, Si forming the substrate and Ti contained in the source gas interact and Si diffuses in the Ti layer, deteriorating the morphology of a TiSix (typically, TiSi2) interfacial layer formed between the Si substrate and the Ti layer. The diffusion of Si into the Ti layer is liable to cause junction leakage when the Ti layer is used for filling a contact hole and affects adversely to electrical connection when the same is used for filling a via hole.
The aspect ratio of contact holes and via holes formed in SiO2 insulating films has increased with the progressive miniaturization of IC chips. Therefore, the Ti film is required to be formed in a satisfactory step coverage.
The present invention has been made in view of the above and it is an object of the present invention to provide a method of forming a Ti film by CVD, capable of forming a Ti film in minute holes formed in an insulating film in a satisfactory step coverage without deteriorating the morphology of a TiSix interfacial layer formed between the Ti film and a Si base.
According to the present invention, there is provided a method of forming a titanium film by chemical vapor deposition in holes formed in an insulating film provided on a silicon base, the method comprising the steps of: loading a silicon base having thereon an insulating film formed with holes into a film forming chamber; evacuating the chamber at a predetermined vacuum; supplying processing gases including TiC4 gas, a reduction gas, Ar gas and SiH4 gas into the film forming chamber; and producing a plasma in the film forming chamber to deposit a titanium film in the holes formed in the insulating film, while said silicon base is heated at a temperature of from 550 to 700xc2x0 C., and while said TiCl4 gas and said SiH4 gas are supplied with a flow rate of the SiH4 gas being from 1 to 50% of a flow rate of the TiCl4 gas, to thereby obtain a silicon-to-insulating film selectivity of not less than one.
The flow rates of the processing gases may be selected so that the silicon -to-insulating film selectivity is three or more. SIH4 may have a flow rate which is from about 0.005 to about 0.25% of the flow rate of all of the processing gases. The temperature of the silicon base during the film formation is preferably from 580 to 700xc2x0 C. H2 gas is used most advantageously as the reduction gas.
The inventors of the present invention made studies to form a Ti film by CVD in a satisfactory step coverage without deteriorating the morphology of the TiSix layer and found that such a Ti film can be formed when the temperature of the substrate is 550xc2x0 C. or more, SiH4 gas is used in addition to TiCl4 gas, H2 gas and Ar gas, and the respective flow rates of those processing gases are adjusted properly.
The deterioration of the morphology of the TiSix interfacial layer formed between the Ti film and the Si base is caused by the interaction of Si and Ti, and the irregular diffusion of Si into the Ti film during the formation of the Ti film. SiH4 gas suppresses the irregular diffusion of Si into the Ti film to prevent the deterioration of the morphology of the TiSix layer. However, since SiH4 gas is a reducing gas, TiSix is produced by the interaction of SiH4 gas and TiCl4 gas and such a reaction deteriorates the step coverage.
To utilize the function of SiH4 to avoid deteriorating the morphology of the TiSix layer without deteriorating the step coverage of the CVD-Ti film, it is effective to increase the silicon-to-insulating film selectivity by heating the substrate at 550xc2x0 C. or more during the film forming process, and to adjust the flow rates of the processing gases, particularly, the flow rate of SiH4 gas, so that the silicon-to-insulating film selectivity is not less than one. The present invention has been made on the basis of such knowledge.
The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings.